The invention relates to an X-ray diagnostic installation for radiography and fluoroscopy with a television installation for the transmission of the X-ray image, comprising a dose rate control device for controlling the dose rate via the voltage and/or the current of the X-ray tube, and comprising a control circuit for controlling the size of the focus of the X-ray tube to which at least one output value of the dose rate control device is supplied. X-ray diagnostic installations of this type are employed for targeted indirect X-ray image intensifier radiographs.
From the German AS 22 45 939 (U.S. Pat. No. 3,991,314 issued Nov. 9, 1976) an X-ray diagnostic installation of the cited type is known, in which a function generator is provided in which various progressions of voltages for the radiograph are programmed-in, in dependence upon the fluoroscopy voltage and the radiographic (or exposure) time, which progressions can be selected by means of keys. From the respective fluoroscopy voltage, the radiography voltage is determined and adjusted in the function generator in dependence upon the selected program. In an associated logic stage, the focus of the X-ray tube, necessary for the radiography voltage, is selected.
Through this arrangement, radiography can proceed immediately following the switching-over from fluoroscopy to radiography. Through the two possible focus sizes, in the case of different loads of the X-ray tube, the focus can be selected which is still permissible given a specified power. The focus size is thereby approximated in steps to the optimum size. What has proven disadvantageous here is that a smallest possible adjustment of the focus size, in general, cannot take place, so that resolution degradations result.